Temperature-induced neutral-ionic transition in 2-chloro-5-methyl-p-phenylenediamine-2,5-dimethyl-dicyanoquinonediimine

A mixed-stack organic charge-transfer crystal of 2-chloro-5-methyl-p -phenylenediamine-2, 5 -dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI), which undergoes a temperature-induced neutral-ionic transition (TINIT), is reported. From infrared spectroscopy, the molecular ionicity (rho) of ClMePD-DMeDCNQI was found to change continuously from p=0.3 at 300 K to rho=0.6 at 100 K. The neutral-ionic interface was crossed without a discontinuity in chi at around 200 K, accompanied by stack dimerization. These features were quite different from the discontinuous change in rho observed for the TINIT of tetrathiafulvalene-rho-chloranil (TTFCA) and tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The transition characteristics in ClMePD-DMeDCNQI are discussed by drawing a comparison with those seen in TTF-CA and TMB-TCNQ.